The field of the present invention is reciprocating air driven devices and the actuators therefor.
Pumps having double diaphragms driven by compressed air directed through an actuator valve are well known. Reference is made to U.S. Pat. Nos. 5,213,485; 5,169,296; and 4,247,264; and to U.S. Pat. Nos. Des. 294,946; 294,947; and 275,858. Actuator valves using a feedback control system are disclosed in U.S. Pat. Nos. 4,242,941 and 4,549,467. The disclosures of the foregoing patents are incorporated herein by reference.
Common to the aforementioned patents on air driven diaphragm pumps is the disclosure of two opposed pumping cavities. The pumping cavities each include a pump chamber housing, an air chamber housing and a diaphragm extending fully across the pumping cavity defined by these two housings. Each pump chamber housing includes an inlet check valve and an outlet check valve. A common shaft typically extends into each air chamber housing to attach to the diaphragms therein.
An actuator valve receives a supply of pressurized air and operates through a feedback control system to alternately pressurize and vent the air chamber side of each pumping cavity through a valve piston. Feedback to the valve piston is typically provided by the position of the shaft attached to the diaphragms which includes valves that alternately vent the ends of the valve cylinder within which the valve piston reciprocates.
The valve piston in such actuators is understood to shift by the selective venting of one end of the enclosing cylinder in which the piston moves. By selectively venting one end or the other of the cylinder, the energy stored in the form of compressed air at the unvented end of the cylinder acts to drive the piston to the alternate end of its stroke. The pressure builds up at both ends of the valve piston between strokes. Pressurized air is allowed to pass longitudinally along the piston within the cylinder to the ends of the piston. Consequently, a clearance has typically been provided between the valve piston and the cylinder.
Under proper conditions, the shifting energy is more than sufficient to insure a complete piston stroke. However, under adverse conditions, the damping or resistance to movement of the piston may so increase that the system may require all available potential energy for shifting of the piston. Under such marginal conditions, all possible energy is advantageously applied to insure operation of the actuator valve.
One mechanism for providing additional energy for shifting is presently included in the devices of the aforementioned patents. Additional compressed air is supplied through passageways to the expanding chamber at one end of the valve piston. The air is gated into the passageways by the location of the piston.
Controlling the clearance between the valve piston and the valve cylinder is also important to maintain sufficient energy. If the clearance becomes too large, excessive blow-by around the piston wastes the energy built up for the shift. With continued shifting, the valve piston wears in the cylinder and builds up excessive clearance. This effect is exacerbated by the pressure forces within the cylinder. There is a differential pressure diametrically across the valve piston between the inlet and the air and exhaust passages on the other side which forces the valve piston against the cylinder. To provide longevity to the actuator, the valve piston has typically been of anodized aluminum and the valve cylinders have typically been made of brass, often lubricated. Polymer materials, which offer advantages in cost of fabrication and not requiring lubrication, have typically exhibited excessive wear when used instead.